Cam mechanism



June 24, 1969 J. P. KIERONSK! 3,451,277

CAM MECHANISM Fri-led June 19, 1967 ll /I0 INVENTOR. JOHN P. KIERONSKI his ATTORNEYS United States Patent US. Cl. 7457 3 Claims ABSTRACT OF THE DISCLOSURE A low stress cam follower for use particularly with drum scroll cam mechanisms has a following surface engageable with the guiding surface in the groove of the drum which is curved in a manner such that the radii of curvature of all points of the following surface are within a range of values between a minimum value which is substantially greater than one-half the width of the cam groove and a maximum value not greater, and preferably somewhat less than the radius of any point along the cam groove. The aforementioned critical limits on the instantaneous radius of points on the following surface of the cam follower provide for significantly reducing contact stresses in the follower as it is guided through the reversal points of the cam groove.

Background 0 the invention This invention relates to cam mechanisms and, more particularly, to cam mechanisms in which high contact or compressive loads are imposed on the follower by virtue of high reversal rates.

In various cam mechanisms, especially those which provide reciprocating output, very high forces between the cam and cam follower are encountered at the reversal points where the reversal rates are high. As the cam follower reverses direction in traversing the cam, it undergoes high deceleration in one direction and then correspondingly high acceleration in the other direction, in other words, high acceleration relative to the cam surface. This high acceleration creates forces that act and react, inter alia, between the cam surface .and the camengaging surface of the follower and result in high compressive or contact stresses on the follower at the point where it engages the cam surface. In the design of a cam mechanism of the type having a follower which is in reciprocating motion, it is frequently the contact stress on the follower at the reversal points that is the limiting factor. If a given movement is desired, then the operating speed must be reduced to a level providing a sufiiciently low reversal rate so that the contact stress on the follower is below the fatigue stress limit of the material.

To demonstrate the problem by reference to a specific instance of it, consider a drum scroll cam used in the yarn traverse guide of a winding machine. The cam drives a yarn guide back and forth across the take-up to wind yarn onto a bobbin to form a roll, commonly called a package. Suppose it is desired for example to wind the yarn at a rate of 3,000 meters per minute onto a 6 in. diameter, 5 /2 in. long bobbin. This rate of winding requires the package to be rotated at, for example, a speed of about 6,270 rpm. at the stage where the package diameter is 6 in. The average velocity of the yarn guide per traverse across the package to wind at this rate is about 550 in./sec. If a 2:1 scroll cam is used, its rotational speed will be two times 6,270 or 12,540 rpm. Assuming that reversal takes place in, as a reasonable, normal value, 10 of deceleration and 10 of acceleration, the time of deceleration or acceleration is 1.328 times 10 seconds. The average acceleration can then be computed from the equation a=v/t (acceleration equals 3,451,277 Patented June 24, 1969 velocity over time), and this turns out to be, for the parameters given, 4.14 times 10 in./sec./sec. or 10,620 gs. Assuming a weight of say one ounce, as a fairly typical value, for the moving parts of the guide, the average force (F :ma) acting between the cam and the follower at the reversal point is over 600 lbs.

The compressive or contact stresses resulting from contact between two cylindrical surfaces having parallel axes can be determined from the Hertz stress equation, which is as follows:

In this equation S is the maximum compressive stress at the contact region in p.s.i.; C and C are the curvatures, in per inch units, of the contacting surfaces; u and n are the Poissons ratios for the respective materials; E and E are the moduli of elasticity for the materials; F is the force in pounds; and L is the length in inches of contact between the surfaces.

Where the cylindrical surfaces in contact are made of like materials having the same moduli of elasticity, where Poissons ratio is taken as 0.3, and where the contacting surfaces are curved in the same direction, the Hertz stress equation can be simplified to:

Conventional cam followers fall generally into two classes, first, a roller follower, in which a wheel is rotatably mounted on a shaft and rolls through the cam groove, and, second, a so-called boat follower, an elongated element that slides along the cam groove. If the compressive stresses for the 600 lb. acting force at the reversal point in the example given above are computed for a conventional roll type cam, as reasonably designed for the yarn winding traverse guide, it will be found that a compressive stress value approaching 400,000 p.s.i. will be obtained. This value is far in excess of the working fatigue stress limits of any practicable materials. Stress calculations for conventional boat type cams will similarly yield stress values far in excess of those that can reasonably be designed for. As a result, it becomes necessary to reduce the speed of the cam, inasmuch as the cam motion is fairly well fixed by the need for uniform winding across the length of the package.

In addition to the problem of compressive stresses at the points of engagement between a boat cam follower and the cam groove, a boat follower also has the further problem of bending stresses that occur whenever the boat follower traverses a curve. In traversing a curve, the follower is supported by the cam surface only at opposite ends; in other words the boat follower spans a segment of the curve while it is mounted on the guide at its center. It is therefore subjected to bending stresses.

Summary of the invention There is provided, in accordance with the present invention, a novel and improved cam mechanism, and especially a cam mechanism embodying a low-stress cam follower. More particularly, the invention is a low-stress, cam follower for use in the type of cam in which the follower is subjected to high compressive loads resulting from high reversal rates such as those frequently encounted in drum cams. Such a cam will usually involve a maximum compressive load at the reversal points, the radius of the cam surface at those points being at a minimum.

The cam follower, according to the invention, is a member adapted to be mounted pivotally on the member to be reciprocated and to slide relative to the cam surface rather than to rotate. It has a maximum transverse dimension slightly less than the width of the cam groove, has a length that is somewhat greater than its width, and will therefore, of course, pivot as needed to keep its longer axis generally orientated along the instantaneous axis of the cam groove. The faces of the follower that engage the cam surface are curved, and its ability to sustain high loads results from maintaining a critical degree of curvature along those faces; specifically, the radii of all points along the cam-engaging faces of the follower are within a range of values between a value substantially greater than /2 the width of the cam groove and a value not greater, and preferably slightly less, than the minimum radius at any point along the cam groove.

Referring back to the Hertz stress formula for computing compressive stresses acting between curved surfaces, it will be observed that the compressive stresses are less as the curvatures (C) of the contacting surfaces are reduced, and therefore significant reductions in compressive stresses are obtained by decreasing the curvature of the cam follower. (The curvature of a curve is the reciprocal of its radius, and thus an increase in the radius means a decrease in curvature.) On the other hand, the curvature of the cam-engaging faces of the follower should not at any point be less than the maximum curvature of the cam surface, inasmuch as this would result in separation of a part of the follower from the cam surface, that is, a spanning of the follower across a part of the cam surface, thus creating other problems.

The benefits obtained with the follower of the invention are manifest. As discussed above, the compressive or contact stresses acting on the follower are substantially reduced for a given cam and operating speed. In practice reductions of several hundred percent in contact stresses can be obtained. Accordingly, with the follower of the invention it is possible to attain higher operating speeds without exceeding the fatigue strength of the material. The follower can be produced relatively cheaply; advantageously, the cam-engaging faces of the follower may be arcuate, which simplifies production procedures and consequently reduces cost.

Brief description of the drawing For a better understanding of the invention, reference may be made to the following description of an exemplary embodiment, taken in conjunction with the figures of the accompanying drawing, in which:

FIG. 1 is a front elevational view of a segment of a drum cam and an exemplary embodiment of the low stress cam follower of the invention; and

FIG. 2 is an end view in section of the cam mechanism of FIG. 1 taken generally along the lines 22 of FIG. 1 and in the direction of the arrows, the ca m follower being shown in full view, however, for greater clarity.

Description of exemplaly embodiment The exemplary embodiment of the low stress cam follower shown in the drawings is suitable for the yarn traverse mechanism in a yarn winding machine. The yarn package is wound up on a rotating bobbin (not shown), the yarn being guided back and forth by the yarn guide (not shown) as it is Wound on the package, thereby creating a helical pattern of predetermined nature. As is well known to those skilled in the art, the motion of the guide is one critical factor in obtaining a uniform package.

The yarn guide is driven by a drum scroll cam, which is designated generally in the drawing by the reference numeral 10. Only a portion of the drum is shown in the drawing, but as is well known in the art, the cam is cylindrical and is formed with an endless cam groove 12 which wraps helically around the body of the drum across the major portion of the cylindrical surface and turns near the ends of the drum through curved reversal portions 12a, only one of which is shown in the drawing. Although the reversal portion 12a is shown as being an are having a radius R, as designated by the arrowed line, the reverse part of the cam groove will not necessarily be arcuate but may be a curve generated by varying radii. In any event the curvature of any point along the reverse portion, that is, the instantaneous curvature, is the reciprocal of the radius at that point. Accordingly, the curvature C of the reverse portion 12a of the cam groove 12 is the reciprocal of the radius R. Regardless of the precise form of reverse portion 12a of the cam groove 12, some point along the path of the reverse curve will have a minimum radius and therefore a maximum curvature. For purposes of illustration, the curve 12a is shown as, and assumed to be, arcuate, having a uniform radius R.

The cam follower, which is designated generally in the drawings by the reference numeral 20, includes a body portion 22, which is adapted to slide in the cam groove 12, having a width very slightly less than the width of the cam groove and a length somewhat greater than its width. As best shown in FIG. 2, the base of the follower body 22 has a curvature to match the cylindrical surface of the base of the cam groove, and the thickness of the body is approximately equal to the depth of the cam groove. Extending outwardly in a radial direction with respect to the cam 10 is a shaft 24, which may be formed integrally with the body 22 or may be a separate element received in a matching hole in the body. In either case the shaft .24 couples the cam follower to a yarn guide (not shown) which is mounted for sliding movement along an axis parallel to the axis of the cam 10. As the cam 10 rotates, the cam follower is reciprocated and guides the yarn back and forth across the rotating yarn package.

As discussed in the section of the application entitled background of the invention, the cam follower 20 is subjected to rapid deceleration as it nears the maximum end point of the reversal zone 12a followed by rapid acceleration in the opposite direction as it leaves the maximum end point. To provide a proper configuration of the yarn as it is wound onto the package, the yarn guide must be reversed in a relatively short period of time, with relation to each cycle of operation of the mechanism, thereby making the deceleration and acceleration of the follower 20 and the moving parts associated with it very high. The value of something over 10,000 gs and the corresponding force over 600 lbs. set forth previously are exemplary. This relatively large force is acting between the contacting surfaces of the cam body 20 and the wall of the cam groove 12.

In the low stress cam follower of the invention, the contact stresses occurring at the point of engagement between the cam and the follower are maintained at practical levels by making the faces 22a and 22b of the follower 20 that engage the walls of the cam groove 12 curved and limiting the curvatures of those faces to a predetermined range of values. More particularly, the radii of the faces 22a and 22b lie between a value substantially higher than one half the width of the cam groove 12 and a value not greater, and preferably slightly less, than the minimum instantaneous radius of the coengaging wall of the cam groove 12. In the embodiment shown in the drawing, the faces 22a and 22b are arcuate, their radius being designated by the arrowed line r, but the curvature of the cam engaging surfaces 22a and 22b of the follower 20 might well be varying. In either case, the instantaneous curvature of any point along the faces should not be less than the maximum curvature of the cam groove -12.

Referring again to the Hertz stress formula, the contact stress acting on the follower 20 at the point of contact of the face 22a and the coengaging Wall of the cam groove 12 is a function of the curvatures of the coengaging surfaces, and it is apparent that maintaining the curvature of the follower 20 within the above-described limits enables the stress to be held to a lower value, all other factors being the same.

The above-described embodiment of the invention is intended to be merely exemplary, and those skilled in the 5 art will be able to make numerous variations and modifications of it without departing from the spirit and scope of the invention. For example, the follower 20 may be provided with a keel on its base to permit the follower to properly traverse a crossover in the cam groove 12.

I claim:

1. In a cam mechanism having a cam groove of a form such that its follower is subjected to high compressive loads resulting from high reversal rates, the cam including a reversal portion having a point where the instantaneous radius of the cam groove is at a minimum, a cam follower receivable in the groove and having a following surface engageable with the wall of the cam groove, the following surface being curved, and the radii of curvature of all points along the following surface being within the range of values between a value substantially greater than one-half the width of the cam groove and a value not in excess of the said minimum radius of the cam reversal portion.

2. A cam mechanism according to claim 1 wherein the 20 two reversal portions, and wherein the cam follower has two curved following surfaces on generally opposite sides thereof, each of the following surfaces having radii of curvature at all points that are within the said range of values, as related to the minimum radius of the respective reversal portions.

References Cited UNITED STATES PATENTS 178,441 6/1876 Houghtaling et al 74-569 2,276,003 3/ 1942 Van Sant 74-57 2,349,314 5/1944 Truesdell 7456 924,509 6/1909 Taylor 74-57 3,353,761 11/1967 Graf 74-57 3,353,760 11/1967 Brehm 74-57 FRED C. MATT-ERN, JR., Primary Examiner.

W. S. RATLIFF, JR., Assistant Examiner.

US. Cl. X.R. 74.-569; 242158.5 

